By Lisa Seachrist

Washington Editor

WASHINGTON ¿ Over the past 30 years the success of clinical transplantation of human organs and tissues to patients suffering from diseases such as kidney and heart failure has ramped up demand for donated organs.

Unfortunately, the supply for such tissues is so limited that approximately half the patients with end-stage disease of a vital organ die while awaiting a suitable transplant. That human tragedy has generated intense interest in using a variety of animal tissue as xenografts into humans.

On Tuesday, the FDA issued a guidance document covering attempts to use nonhuman primate xenografts in humans. It highlighted the risks that must be addressed before nonhuman primate xenotransplantation can be attempted in human beings.

The agency recommends that all novel xenotransplantation protocols be evaluated by an appropriate federal xenotransplantation advisory committee, such as the Health and Human Services Secretary¿s Advisory Committee on Xenotransplantation (SACX), which is now under development.

Finding that current scientific data is inadequate to assess risks to both transplant recipients and the general public, FDA is requiring clinical protocols proposing nonhuman primate xenografts be submitted only after sufficient scientific information can address those risks.

In addition, the agency is calling for public discussion to address all the issues raised by xenotransplantation.

The FDA is particularly concerned that xenotransplantation poses unique risks not only to the person receiving the organ, tissue or live cells from another species, but to the general public as well. Viruses and other infectious agents such as the prions that cause diseases like bovine spongiform encephalopathy and Creutzfeldt-Jacob disease could be transmitted from the nonhuman primate xenograft to the recipient.

As a result the recipient, who is taking powerful immunosuppressant drugs, may be susceptible to transmission of infectious agents that otherwise wouldn¿t be transmitted from animal to human. In addition, should the agent establish an active infection, the recipient of the xenotransplant may then infect the general public.

Because most nonhuman primates are only a generation or two out of the wild, it can be very difficult to identify infectious agents that potentially are transmissible to humans. Other species raised under more controlled conditions may not raise this level of concern.

The guidance goes into effect immediately. However, the agency is requesting comment on certain measures that may reduce the risk of transmitting disease between species. For example, the agency notes in the Federal Register that extensive preclinical engineering, enclosure of the product in a semi-permeable barrier, and/or the use of well-characterized cell lines could potentially provide greater control of infectious disease risks.

In addition, the agency will be releasing other guidances to deal with xenotransplantation issues in other species, but the agency won¿t be issuing species-specific guidances.